The use of in-ground vapor collectors to obtain samples of subsurface gases is well known. For example, they can be used in geological explorations for oil, natural gas and geothermal resources. Another example is in the collection of gases emanating from pollutants in the ground from such sources as chemical wastes, accidental spills, tank or piping leaks and the like.
These soil gases are often present in very low concentrations or in soil conditions that make it difficult to obtain a reliable results using active gas collection techniques. Passive vapor collectors accumulate gases over extended periods of time and can be left in place until sufficient sample is sorbed by them so that reliable measurements of the gases can be made.
Active collection methods are distinguished from passive collection methods by the means by which the sampled gases are conveyed to collectors or to analytical instruments. In active collection methods soil-gas samples re drawn from the in-ground sampling location and transferred by pumping them through pipes or tubing connected to collectors or analytical instruments which are usually above-ground. The samples of this method are transferred at relatively high rates over relatively short sampling periods. In passive collection methods the collectors are sited directly at in-ground sampling locations without connection to external equipment. Samples are accumulated in situ at much lower rates and over relatively long periods of time by diffusion of soil-gases into the passive collectors.
It can be seen from the application listed above that the collectors may be placed in locations and environments that vary extremely in their topography, weather, soil types and soil conditions. These variations must be faced as a matter of fact and problems associated with them overcome. Of particular concern are those variations which influence the performance of the sorbents and those which influence in-ground insertion and retrieval of the collectors.
In order for passive vapor collectors to function effectively their sorbents must be kept clean and free of contaminating materials. In many locations the presence of water will adversely affect the performance of the sorbents; either by occupying sorbent sites itself when it contacts the sorbent or by exposing the sorbent to water-borne contaminants which then occupy the sorbent sites.
The in-ground insertion and retrieval of passive vapor collectors also expose them to risks that may adversely affect their performance. There is a risk of physical damage to the passive vapor collectors during the insertion and retrieval steps, in particularly, during the excavation step necessary to recover the collector for further work. There is also a risk of sorbent contamination during the insertion and retrieval steps. This risk is associated with exposure of the collector to the surface atmosphere during the time required to carefully emplace the collector and the much more lengthy time required for careful excavation and recovery of the collector.
There is another risk of sorbent contamination which must be considered. This risk is associated with the size and depth of the hole required for emplacement of the collector. The collector should be located at a depth that prevents influence by the surface atmosphere in a hole created with minimum ground disturbance. The wider and shallower a hole is, the more difficult it becomes to reliably seal it against surface influence.
It can be seen then that a system that provides protection from contamination and physical damage to the collectors, minimizes ground disturbance, facilitates in-ground insertion and retrieval, and eliminates the need for excavation to recover the collectors would be very desirable.